Two Roads Diverged...
The Aliens came in the year 2047. It looked like a war was going to break out. China demanded massive food aid or they'd nuke America, Canada, Australia, and Argentina. Russia was in full collapse. Egypt and Ethiopia were locked in a brutal war over the waters of the Blue Nile. Pakistan had invaded Kashmir and threatened India with nukes if the challenged them. China had already shut off all the rivers of the Tibetan plateau. The Aliens said, "STOP!"
As they had the spaceship gunboats that proved they could destroy anything on Earth, people listened. Sure, some people tested their patience. They died quickly.
The Aliens ordered humanity to leave the Earth. This brought howls of fury and despair.
The American President calmly asked "How?"
The aliens then gave the President the Passkey.
Then the President opened the Doors!
Basically, in the middle of the 21st century humanity was given a series of nexial gates that lead out to other Earth-like worlds. At the same time humanity was given an eviction notice. And the proof the eviction could be enforced.
Humanity marched away from the Earth. They had no choice.
Different nations and cultural groups were given their own worlds or territories. The USA split up into several different parts by political faction. This was a fairly normal thing.
A few decades later the
Doors closed.
Now, several centuries later on a world settled by the Social Democratic factions of the USA and several allied nations, Humanity has once again been contacted by the Aliens.
The Aliens explained that Earth was a rare planet. Life evolved independently there. In most of the universe predatory forces from higher dimensions inhibit the development of life. Earth's ecosystem was so vigorous the Aliens used Earth life forms to seed Earth's galaxy with life. This explained why all the worlds Humanity went to had life forms that seemed to diverge from Earth life no later than the mid
Pleistocene. The worlds humanity had been allowed to go to had been preselected for this.
Now, after the Earth's ecosystems had healed, humanity would be allowed to visit their ancient home again. But first Humanity must build an organization to hold the Earth in trust.
Thus the
Doors will be opened again in order for Humanity to find its divergent tribes and nations, As this world has the
Passkey it is their job to find everybody and organize the creation of the Trust.
Think of this as a version of
Stargate with a very
Lovecraft twist. If you have a copy of any of
Gurps: Chulhupunk or
Eldrich Skies or
Call of Cthulhu, strip mine them for ideas.
Ideally, the PCs will be small groups of explorers with a mixture of survival and cultural skills. Remember they've got to travel through a network of Nexial gates, across wildernesses, to long isolated human groups, and make contact with their leaders.
The world the PCs are presumed to come from, New Avalon, has about the technology you'd expect to find in the
Transhuman Space in the year 2150. Technology was significantly slowed down by the disruptions of Humanity's dispersal. Assume that New Avalon is the most generally advanced planet, but other worlds may be advanced over New Avalon in narrow fields. Some human societies may have collapsed after the removal from the Earth. Also, many human societies are antidemocratic, xenophobic, or simply aggressive. Friendly or even non-violent welcomes aren't guaranteed.